Producing a mirrorlike finish on aluminum



Patented Mar. 23, 1954 1 PRODUCING A Mnmonuxn FINISH N ALUMINUM i Eugene R. Du Fresno, Chicago, and John R. Swihart, Chicago Heights, 111., assignors to The Diversey Corporation, a corporation of Illinois No Drawing. Application March 10, 1951, Serial No. 215,031

15 Claims. (Cl. 4142) This invention relates to the production of A combination of sodium nitrate and sodium a mirrored surface on aluminum, a bath for prochlorite which gave good results was as follows; ducing it, and the products so produced.

In the ordinary commercial production of alu- 1 minum, the material, after fabrication, is custo- 5 marily etched to remove surface scratches and 20% Q produce a satin finish. It has now been dis-- Sodium gluoonate covered that by using an etching bath composed 50% sodlum f of sodium gluconate, sodium hydroxide, and so- 20% sodlum chlol'lte dium nitrate, the resulting finish instead of being 10 satin is a highly mirrored reflecting surface.

Acid baths for producing a mirrored surface have heretofore been suggested, but they are expensive, require special tanks, require high concentrations of acids, and there is considerable This was used at the rate of 16 ounces per gallon at a temperature of 205? F. for 1% minutes on 2 S aluminum.

The same mixture used with only 1% sodium gluconate was also satisfactory.

loss due to drag out. 2

The present invention utilizes caustic soda baths including an oxidizing agent and a water The following composition employed consider soluble saturated aliphatlc polyhydroxy ably less sodium nitrate and sodium chlorite than pound of the class consisting of sorbitol, gluconic 20 the preceding but likewise gave good results: acid its salts and lactones and mucic acid its salts and lactones. 2.5% Sodium gluconate Glycerine and gum arabic have been found de- 62 ti d sirable additives in that they keep the accumuladi t t tion of smut at a minimum, and thus assist in 25 105% di hl t the achievement of a better polish.

The preferred oxidizing agent is sodium ni- The composition gave good results using a contrate. Sodium chlorate and sodium nitrite are centration of 14 to 32 ounces per gallon of solualso quite satisfactory. Sodium perchlorate and tion. It was used at 205 F. for 1 minutes. sodium hypochlorite are less satisfactory, but at Good polishing results were obtained using only have some polishing effects. A combination of 16 ounces per gallon at 160 F. Some results were sodium nitrate and sodium chlorite gave very Obtained using y 4 Ounces p gallon, but the good results with considerably less total of the results W not up 50 normal Commercial two compounds than gave the best results with q t sodium nitrate alone. 3

The preferred composition is one part of so- A bath using 25% caustic soda 8% sodium dium gluconate, six parts of caustic soda, and gluconate, and 67% sodium nitrate employed h to parts of sodlum mtmte; The at the rate of 16 ounces of the composition per pos1t1on is employed 1n awater so1ut1on,normalgallon of solution gave good results at F 1y using from six to eight ounces per gallon of 40 Bath 1 was even better. caustic soda. That is, enough of the composition is employed to give a concentration of six to 4 eight ounces of caustic soda per gallon of solu- T same b th employing 30% sodium nitrate, ti n- Th in a mp n e p of l ni caustic soda, and 4 92; sodium gluconate pound of sodium gluconate, 6 pounds of caustic wa a1 satisfactory.

So and 8 p u of Sodium nitrate, 1520 When used on a series of aluminum parts, the Ounces a e u d p gallon of Solution baths continued to give a polishing action so The polishing bath may be employed in the long as there was available caustic for reaction cold or heated. The polishing effect becomes with the metal. quite evident at 160 F. or higher, but notice- Other oxidizing agents which of themselves able action has been produced at room temperwould not produce a polishing action, tended to ature. make any smut formed more adherent. While The aluminum-article is normally left in the smut is undesirable, so long as it is present at bath for aminute ortwo or until a maximum all, it is better to have it adherent; so that these polishing effect on the surface has been achieved. 56 oxidizing agents also have a value in the process.

The following formulation gave good results when employed at 16 ounces per gallon of solu tion at 205 F. for 1 /2 minutes:

20% caustic soda 8% sodium gluconate 8% gum arabic 44% sodium nitrate 20% sodium chlorite This particular bath required a temperature in excess of 150 F. to give a good polishing action.

In general with the various solutions, the time of treatment varies from 1%; to 2 minutes. If too long a time is used, gas streaking is likely to occur.

Using a reflectometer with the normal satin finish resulting from the standard caustic and nitrate etching bath as zero, the results obtained by adding gluconate were 85; and using a combination of nitrate and chlorite with added gluconate, the reading was 100.

Best results have been obtained upon 2 S aluminum. Sodium gluconate, glucono delta lactone, glucoheptonic lactone, sorbitol, and mucic acid all gave good results on this form of aluminum. 24 S, 61 S and 75 S aluminum produced fair polish.

20% NaOH sorbitol 50% sodium nitrate sodium chlorite NaOH mucic acid sodium nitrate sodium chlorite Likewise the glucono delta lactone and glucoheptonic lactone were substituted in the same formula under the same conditions with good results.

In treatments of aluminum with caustic materials where a chemical reaction with the metal occurs, a dark smut is formed. Polishing aluminum in the described baths often results in a similar condition. This smut will obscure the full luster obtained on the metallic surface. Such smut can be removed mechanically by rubbing or by acidic treatment in dilute solutions of nitric, sulfuric and chromic acids or combina tions of these acids. These post polishing treatments remove the interfering substances forming the smut and permit the full luster obtained from the polishing bath to be seen.

The foregoing detailed description is given for clearness of understanding only and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art.

We claim:

1. The method of forming a reflecting surface on aluminum which comprises immersing it in a caustic soda bath including a small proportion of sodium gluconate and .a large amount of a dissolved oxidizing agent. I,

2. The method as set forth in claim 1 inwhich 4 the temperature of the bath is at least F.

3. The method as set forth in claim 1 in which the bath contains water, approximately 1 part sodium gluconate, approximately 6 parts caustic soda, and 8 to 10 parts of sodium nitrate.

4. The method as set forth in claim 1 in which the oxidizing agent is sodium chlorate.

5. An aluminum etching bath comprising water, sodium gluconate, caustic soda and an oxidizing agent.

6. An aqueous solution containing 6 to 8 ounces per gallon of caustic soda, approximately one part of sodium gluconate for 6 parts caustic soda and 8 to 10 parts of sodium nitrate for each 6 parts caustic soda.

7. An aqueous solution containing 6 to 8 ounces per gallon of caustic soda, approximately one part of sodium gluconate for 6 parts caustic soda and 8 to 10 parts of sodium chlorate for each 6 parts caustic soda.

8. The method which comprises immersing an unpolished, fabricated aluminum object for a few minutes in a bath containing 6 to 8 ounces of caustic soda. per gallon of water, one part sodium gluconate for 6 parts caustic soda and 8 to 10 parts of sodium nitrate for 6 parts of caustic soda whereby a mirrored surface is pro duced on the aluminum object.

9. The method of treating aluminum to pro duce a mirrordike surface thereon which comprises immersing an aluminum surface in a caustic soda bath containing an oxidizing agent of the class consisting of sodium nitrate, sodium nitrite, sodium chlorate, sodium hypochlorite, and sodium perchlorate and a proportion of a polishing compound of the class consisting of polyhydric alcohols of the formula CH2OH(CHOH) aCHZOH where a is at least i, salts and lactones of polyhydroxy monohasic acids having the general formula COO-H(CHOH) cCHZOH wherein c is at least 4, salts and lactones of polyhydroxy dibasic acids having the general formula COOH(CHOH) dCOOH CI-120111 CHOH) (X12033 where a is at least 4, salts and lactones of polyhydroxy monobasic acids having the general formula COOH(CHOH) cCHZOH wherein c is at least 4, salts and lactones of polyhydroxy dibasic acids having the general formula COOH CHOH aCOOI-I wherein (Z is at least 2.

11. The method as set forth in claim 9 in which the oxidizing agent includes sodium chlorite.

12. The method as set forth in claim 9 in which the polishing compound is sorbitol.

13. The method as set forth in claim 9 in which the polishing compound is mucic acid.

14. The method as set forth .in claim 9 in which a compound of the class of glycerine and gum arabic is included in proportion to control smut.

15. A bath for polishing aluminum comprising caustic soda containing an oxidizing agent of the class consisting of sodium nitrate, sodium nitrite, sodium chlorate, sodium hypochlorite, and sodium perchlorate and a proportion of a polishing compound of the class consisting of polyhydric alcohols of the formula hydroxy monobasic acids having the general formula. COOH(CHOH)CH2OH wherein c is at least 6 4, salts and lactones of polyhydroxy dibasic acids having the general formula COOH (CHOH) aCOOH wherein dis at least 2.

EUGENE R. DU FRESNE. JOHN R. SWIHART'.

References Cited in the file of this patent UNITED STATES PATENTS Number 

1. THE METHOD OF FORMING A REFLECTING SURFACE ON ALUMINUM WHICH COMPRISES IMMERSING IT IN A CAUSTIC SODA BATH INCLUDING A SMALL PROPORTION SODIUM GLUCONATE AND A LARGE AMOUNT OF A DISSOLVED OXIDIZING AGENT. 